THE SMALL RIBOSOMAL-SUBUNIT RNA ISOFORMS IN PLASMODIUM-CYNOMOLGI

We report the isolation, characterization and analysis of the small subunit rRNA genes in Plasmodium cynomolgi (Ceylon). As in other Plasmodium species, these genes are present in low copy number, are unlinked and form two types that are distinct in sequence and are expressed stage specifically. The asexually expressed (type A) genes are present in four copies in the Ceylon and in five copies in the Berokstrain. Surprisingly, the sexually expressed (type B) gene is present in a single copy. The vast majority of the differences between gene types is confined to the variable regions. The pattern of divergence is different from that observed in Plasmodium berghei or in Plasmodium falciparum. Analysis of the small subunit rRNA sequences of P. cynomolgi, P. berghei and P. falciparum, indicates that the mio gene types do not evolve independently but rather interact (through gene conversion or some form of recombination) to such an extent as to erase whatever stage-specific sequence signatures they may have had in the last common ancestor.